1. Field of the Invention
The present invention relates to a drawn strand filled tubing wire for use in medical applications and in particular to such wire where there is a need to apply an electrical voltage to human tissue.
2. Description of the Related Art
Implantable devices used in the medical field for applying electrical voltage are customarily meant to stay implanted for several years. Such devices are used as pacing leads, for example. These medical devices must possess several characteristics including electrical conductivity, corrosion resistance, and biocompatibility. The medical devices generally need to be flexible for snaking through arteries, for example.
Drawn filled tubing wire is a type of wire that has been used extensively in medical devices. This wire includes an outer shell that is filled with an electrically conductive material. While the materials used for the outer shell are strong, they tend to be susceptible to corrosion when contacted by body tissues and fluids. Therefore, drawn filled tubing wire for medical use is customarily coated with an insulating material such as silicone to prevent contact with human body tissue. Pacing leads for applying an electric potential to the heart usually comprise two or three drawn filled tubing wires. Such leads are described in U.S. Pat. Nos. 5,716,391, 5,755,760, 5,796,044, and 5,871,531. A portion of the wires in such leads is generally encased within a biocompatible material such as platinum, tantalum filled platinum, tantalum filled platinum-iridium, or the like to allow an electrical voltage to be applied from the wire to the desired tissue area. A problem with such biocompatible materials is that they have insufficient strength and have limited electrical conductivity, and therefore must be combined with the wire.
Referring to FIGS. 1A and 1B, a prior art pacing lead is shown for use in medical applications. Implantable cardio defibrillator (ICD) 20 is used for sensing electrical activity in the heart and for delivering a shock if heart activity slows or stops. ICD 20 is implantable and has flexible, elongated conductive lead 26 (FIG. 1B) with electrical connector 22 extending from one end thereof to plug into control 24 for controlling ICD 20 and for providing the electrical supply. Control 24 is implanted just beneath the skin, often in the chest or abdomen.
Lead 26 is constructed from two or three electrically conductive wires 27 such as wires having an alloy exterior tube filled with highly conductive silver, for example. Each wire 27 is substantially covered with insulating material 29. Lead 26 is then substantially covered with insulating material 28. At two locations along lead 26, coils 30 are located which are made from a biocompatible material such as platinum, tantalum filled platinum, tantalum filled platinum-iridium, or the like. Coils 30 are secured to individual wires 27 of lead 26 by any suitable process including laser welding. The portion of wire 27 in contact with coil 30 has insulating material 29 removed to allow for the welding process. These coils 30 form the contacts which engage the heart tissue at specific locations to deliver an electrical voltage, when control 24 senses the need to deliver such voltage.
The interface between insulating material 28 and coils 30 must be hermetically sealed to prevent fluids from contacting wires 27 of lead 26 and causing corrosion and possible eventual failure of the ICD. Problems exist in that the achieving a hermetic seal of a polymeric material and a metal is difficult and costly. The bond may be susceptible to corrosion and bodily fluid leaking into the area between coil 30 or insulating materials 28, and wires 27 of lead 26. In addition, the materials used to form coils 30 are very flexible and may be easily damaged simply from handling the coils. The welding process between wires 27 of lead 26 and coils 30 is a further step in the manufacturing process which increases the cost of production of ICD 20.
In the medical device industry, leads are used to transmit an electrical voltage from an electrical supply source to an area in a human body. The lead interfaces with tissues in the body so that an electrical signal may be introduced to a particular area of the body. Such leads may be implanted in a patient at any location in the body where the electrophysiology needs to be monitored and/or artificially altered. Specific applications may be implantable defibrillators or pacing leads. The leads may also be used for pain relief or pain suppression in the back or spine relating to diseases such as Parkinson's disease. The lead may be further implanted in the stomach to subside hunger pains. For patients with neurological damage, the leads might be used to replace the nerve and act to transmit electrical signals from one place to another in the body. These devices are most certainly used in humans however, they are not limited to humans and may be adapted for use in animals.
The devices are designed for long term implantation and must have several properties including resistivity, corrosion resistance, radiopacity, reliability, stiffness, fatigue life, weldability, MRI compatibility, and biocompatibility. Other characteristics of the device include a predetermined ultimate tensile strength, Young's modulus, level of inclusions, fracture toughness, and percent elongation. In addition, the types of materials used, the construction, and the cost of manufacturing the device are all factors.
It is therefore an object of the present invention to provide a pacing lead with improved wires which eliminate the need for conductive coils.
It is therefore a further object of the present invention to reduce the risk of corrosion of the pacing lead.
It is therefore another object of the present invention to improve conductivity and flexibility of the pacing lead.